This invention relates generally to magnetic tape transports, sometimes referred to as tape units, tape drives, tape decks, etc., for use with a replaceable tape assembly known as a tape cartridge or cassette, and particularly to those used as a subsystem to enable a host system to obtain access to data on the magnetic tape. More particularly, the invention concerns a method of, and means for, accurately positioning a read/write head unit with respect to a plurality or multiplicity of longitudinal tracks on the tape in such tape transport.
Digital cassette tape transports, particularly to those operating in streaming mode, as contrasted with start/stop mode, have come to find extensive use as peripherals of computer systems. The current trend in such tape transports is toward greater tape capacity. This trend best manifests itself as an increasingly greater number of tracks on the tape. Such an increase in the number of tracks necessarily incurs a more and more decrease in the width of each track as well as in the spaces between the tracks; indeed, one of the latest high capacity tapes has as many as twenty nine tracks, compared with eight tracks on standard digital audio tape. Each of the twenty nine tracks has a width of only 0.15 millimeter. Thus the exact placement of the head unit on tape tracks is becoming increasingly more difficult with an increase in the number of tracks.
Inserted in a tape transport, a tape cartridge is mechanically positioned therein, and the tape extends along a predefined transport path defined by guide members. The final position of the tape cartridge in the tape transport, or of the transverse position of the tape, relative to the head unit, is subject to variations that can be tolerated in the case of eight track audio tape, but not in the case of high track density tape, if the position of the head unit is to be controlled with respect to the tape tracks merely by, for example, a stepper motor in combination with a lead screw mechanism.
U.S. Pat. No. 4,786,984 to Seeman reflects a conventional attempt at more stringently controlling the transverse position of the head unit relative to high track density tape despite the unavoidable variations on the final position of the tape cassette, and of the tape itself, relative to the tape transport. Seeman teaches to detect a tape edge as a reference location of the head unit, from which the unit may be moved to and positioned on any track for reading and writing.